The present invention relates to a refrigeration system and in particular to a refrigeration system and compressor wherein hot discharge gas and liquid refrigerant are bypassed to the suction side of the compressor in order to provide for modulation of cooling capacity. The refrigeration system is particularly adapted for use in a vehicle air conditioning system.
In automotive air conditioning systems, if the ambient temperature is quite high, the air conditioner refrigeration system normally operates continuously in order to maintain the desired temperature within the vehicle. However, under cooler ambient temperature conditions, continuous operation of the compressor provides too much cooling capacity so that the temperature of the evaporator decreases below freezing and ice forms on the evaporator fins. Such undesirable freezing up of the evaporator is even more likely to occur under high humidity conditions.
One known way to prevent evaporator freeze up and modulate the capacity of the refrigeration system is to cycle the compressor off when the evaporator becomes too cold. A disadvantage to this technique is that the repeated cycling on and off of the compressor is likely to produce accelerated wear of the clutch plates and other parts of the compressor due to the rapid acceleration of the compressor moving parts when the clutch plates are engaged. In vehicles with small engines there is a further disadvantage in that the intermittent loading and unloading of the engine as the compressor cycles on an off causes a noticeable variation in engine speed.
Another prior art technique for modulating the capacity of the refrigeration system is to employ a compressor having variable displacement. In a swash plate compressor, the pitch of the swash plate can be varied to thereby change displacement. In multi-cylinder compressors, one or more cylinders can be blocked to thereby partially unload the compressor. However, the mechanisms for displacement variation are typically complicated and may require sophisticated controls.
One known possibility is to bypass a portion of the discharge gas to the suction side of the compressor, but this normally causes a rise in the temperature of the compressor to unacceptably high levels. Another known possibility is to bypass a portion of the liquid to the suction side of the compressor in response to suction gas temperature in order to maintain a substantially constant predetermined super-heat of the suction gas entering the compressor.
Although bypassing of liquid refrigerant from the condenser or hot discharge gas from the discharge side of the compressor will affect the capacity of the compressor, such prior art attempts at providing capacity control have not been effective in providing smooth control over the full range of ambient temperatures experienced by a vehicle air conditioning system.